It's also happened in two other elections with two major candidates: In 1888, when Benjamin Harrison defeated popular-vote-winning president Grover Cleveland; and in 1876, when Rutherford Hayes scraped by Samuel Tilden after disputed returns in four states. (We're not counting the multi-candidate election of 1824, decided by the House of Representatives.)

If it happens again, expect to hear more calls to abolish the Electoral College -- but don't expect anything to change.

Smaller states would likely block any effort to eliminate the Electoral College because it gives them the prospect of power.

In this election, for example, battleground states include New Hampshire (four electoral votes), New Mexico (five), and Nevada (six) -- any of which could make the difference in a tight election.